How can I automatically resize the root partition on first boot?

Question

I'm preparing an image for download based on Raspbian. I can resize the partition manually using raspi-config. However, I'd prefer this to be automatic during first boot (maybe with a timeout for cancelling if such is possible).

Apparently BerryBoot is able to do this automatically- would appreciate a hint how this is done.

Answer 1

1. make yourself a script that deletes or disables itself after first run (init for background or .profile for foreground)

grep -v custom_script.sh ~/.profile > ~/.profile.new && mv ~/.profile.new ~/.profile

2. extract the bits you want from raspi-config (line 36 to to 116)

> file $(which raspi-config)
/usr/bin/raspi-config: POSIX shell script, ASCII text executable